The present invention generally relates to vehicles and more particularly to a front wheel drive, rear steer vehicle such as a motorcycle or bicycle.
Prior vehicles have ranged in design from conventional rear wheel drive, front wheel steer vehicles to unique, limited use front wheel drive, rear wheel steer vehicles and many variations in between. Conventional rear wheel drive, front wheel steer single track (i.e., two wheel) vehicles have inherent limitations stemming from the fact that while the front wheel is attempting to take the vehicle into a turn by being steered in the direction of the turn, the rear drive wheel is attempting to drive the vehicle in a direction generally tangent to the turn since this wheel is locked in that direction.
For this reason, in order to drive the vehicle through a "tight" turn at high speed the operator must allow the rear wheel to slide sideways and outwardly in a direction generally tangent to the turn. As a result, even skilled operators of such vehicles risk losing control of the vehicle during a tight turn.
The alternative to the practice of allowing the rear wheel to slide out during a turn is to reduce the speed of the vehicle during the turn such that the front wheel may be turned more sharply and the operator need not operate the vehicle such that the rear wheel slides out of the turn and control is lost. However, this is an unacceptable alternative for many people and especially when the vehicle is used competitively or where overall speed is otherwise important.
If these difficulties could be overcome, speeds could be increased without any safety disadvantages, or the same speeds maintained but with increased safety margins.
In the area of single track vehicles, numerous designs have addressed the short comings of conventional rear wheel drive, front wheel steer vehicles with regard to their ability to maintain speed and balance while negotiating turns. One proposed solution addressing the problem of turn instability and low turn speeds has been to drive the front wheel and provide a steering mechanism for the rear wheel. In U.S. Pat. Nos. 4,650,021 and 4,662,469, each issued to Matsuda et al., a rear wheel drive motorcycle is disclosed having steering mechanisms for both the front and the rear wheels which simultaneously steer both wheels during a turn. Each of these patents disclose similar, relatively complex linkages for causing simultaneous steering of the front and rear wheels. Also, there is no indication given in either of these patents that the front and rear wheels exactly, or even substantially, track each other during a turn. That is, there is no indication that the rear wheel follows along substantially the same path as the front wheel during a turn. This is important since such tracking allows the power being transmitted to the drive wheel to be most completely used to move the vehicle in the direction of the turn without having to overcome the forces being generated by one wheel moving in a direction other than the direction of the turn.
Other past designs have incorporated a front wheel drive mechanism in combination with front and rear wheel steering mechanisms. In U.S. Pat. No. 5,042,608 to Horiike et al. (Horiike '608), a front wheel drive motorcycle is disclosed which is said to be able to make a turn at high speed. The motorcycle disclosed in Horiike '608 makes use of a relatively complex design which includes a joint shaft for allowing the front frame of the motorcycle to tilt and a steering shaft for allowing the rear wheel to turn. The axis of the joint shaft extends rearwardly and downwardly such that it contacts an imaginary point on the road surface in front of the rear wheel and the axis of the steering shaft extends either in a vertical direction or rearwardly and downwardly such that it contacts an imaginary point on the road surface behind the rear wheel. As the rear wheel banks into a turn, the joint shaft tilts to cause the front wheel to turn simultaneously in the same direction.
In U.S. Pat. No. 5,076,388 to Horiike et al. (Horiike '388) another motorcycle having steering mechanisms for both the front and rear wheels is disclosed. Horiike '388 further discloses that either the front or rear wheel of the vehicle may be driven by a power unit such as an engine. Various embodiments of Horiike '388 are even more complex than those disclosed in Horiike '608 in that the steering mechanisms are driven by motors operated in response to sensors which sense the banking angle of the motorcycle during a turn. Moreover, like the patents issued to Matsuda et al., and described above, there is also no indication in either Horiike '608 or Horiike '388 that their designs cause the front and rear wheels of the vehicle to substantially track each other in a turn.
Another patent, U.S. Pat. No. 2,101,057 issued to Fuller, discloses a front wheel drive rear wheel steer vehicle having a center of gravity which is located forward of the midpoint of the wheel base. In other words, the center of gravity of the vehicle is located closer to the drive wheels than to the steered wheel or wheels. Fuller states that this eliminates the tendency to skid when braking or rounding corners. Fuller does not, however, recognize or disclose the further advantages of having a steering mechanism which allows the front and rear wheels of the vehicle to track each other during a turn. In the Fuller design, the rear wheel can leave the lane in which the front wheels are located during a turn with the result that skidding is, in fact, possible or that energy is otherwise inefficiently used during the turn.
Another disadvantage of prior vehicles relates to the rate of change in the turn radius of the vehicle in response to the manipulation of the steering head. Conventional steering mechanisms are undesirably sensitive to steering inputs in tight radius turns, but less sensitive in larger radius turns. Stated another way, in a conventional steering mechanism the rate of change of the steering head angle varies in a nonlinear fashion with respect to the corresponding rate of change in the turn radius of the vehicle. For example, in such a conventional mechanism, the rate of change in the vehicle's turn radius increases as the steering head angle increases. In tight turns, that increasing ratio of turn radius change to steering head angle change makes the vehicle very sensitive to steering angle changes and thus more difficult to control; smaller steering inputs cause larger turn radius variations than the same steering angle change when the vehicle is moving in a straight line or through a very wide turn. This is most dramatically illustrated in a conventional rear wheel drive, front wheel steer vehicle such as a motorcycle or bicycle. When the steering head is turned to an angle approaching a full "lock" position, i.e., the position at which the front wheel cannot be further turned, very minor changes in the steering head angle cause large changes in the turn radius of the vehicle. This situation is exacerbated when the vehicle is driven over rough terrain, which makes it more difficult for the rider to precisely control steering angle.
Thus, greater sensitivity to steering input, coupled with rider jostling due to terrain, makes the conventionally steered vehicle hard to control. As a result, it would be advantageous to provide a steering mechanism for a vehicle in which changes in the turn angle of the steering head correspond more accurately to changes in the turn radius of the vehicle. In other words, it is desirable to produce a vehicle which is more stable to operate during tight turns.
One further disadvantage of a conventional steering mechanism for a single track vehicle arises from the use of a typical handle bar attachment to a steering head. Considering a conventional bicycle, for example, as the handle bar is turned from its position transverse to the front to rear center line of the bicycle, it becomes more closely aligned with the centerline. When the handle bar is transversely positioned, or when the vehicle is moving straight ahead or is in only a slight turn, it is easier to grasp and control than when turned to a position more aligned with the bicycle's center line. When turned, one end is near the rider, cramping movement, while the other end is forward and away from the rider, requiring the rider to reach out and hold it.
Thus, when the handle bar is turned, as in a tight turn, it presents and awkward situation to the rider and is harder to control. Also, when coupled with the previously mentioned difficulties of increased turn radius sensitivity to steering angle movement, and the difficulties of turn control over rugged terrain, the conventional bicycle handle bar presents significant disadvantages and inherent steering difficulties.
Accordingly, one objective of the present invention has been to provide a front wheel drive, rear wheel steer, vehicle which is able to make turns at higher speeds with the same level of safety as was heretofore possible or at lower speeds with a higher level of safety than was heretofore possible.
It has been another object of the invention to provide a vehicle able to climb hills and to make turns while climbing such hills at greater speed than was heretofore possible by allowing more positive traction between the drive wheel and the road or path being climbed.
It has been still another object of the invention to provide such a vehicle with a steering mechanism which allows the front and rear wheels of, for example, a single track vehicle to follow substantially the same path or to substantially track each other around a turn and thereby cause less slippage and/or friction between the tires of the vehicle and the road and allow more power to be transferred between the tires and the road in the direction of the turn.
It has been yet another object of the invention to provide an improved steering mechanism for a single track vehicle and which reduces sensitivity of turn radius to steering angle changes when the vehicle is in a tight turn.
It has been still a further object of the invention to provide an improved steering mechanism for a vehicle which provides for more positive control of the handle bar for all steering angles.